Heavy Water

Heavy water is chemically the same as regular (light) water,
but with the two hydrogen atoms (as in H2O) replaced with
deuterium atoms (hence the symbol D2O). Deuterium is
an isotope of hydrogen; it has one extra neutron. Thus the deutrium atom
consists of one proton and one neutron in the atomic nucleus and
one orbiting electron. It is the extra neutron that makes heavy water
"heavy", about 10% heavier in fact.

Heavy water is essential to the operation of Canada's CANDU
nuclear power reactors; used as both a moderator and a heat transfer
agent. The function of the moderator is to slow down the emitted
neutrons, which increases the fission reaction rate, thus
enabling a sustained chain reaction. Ordinary water is also a
good moderator, but it absorbs neutrons, which is why light water
reactors must use enriched fuel. The use of D2O in
CANDUs makes it possible to use non-enriched natural uranium.

Bruce heavy water plant 'A' (now decommissioned). Bruce NGS B is in the
background and Douglas Point is to the right.

Heavy water is produced at Ontario Hydro's heavy water plant 'B'
at the Bruce Power Development in Tiverton, Ontario. The heavy water
is not manufactured, but rather it is extracted from the quantity
that is found naturally in lake water. The water is separated
through a series of towers, using hydrogen sulphide as an agent.
Owing to AECLs CANDU programme, Canada is the world's supplier
of heavy water.

The heavy water for the Sudbury Neutrino Observatory (SNO)
is on loan from Atomic Energy of Canada
Limited (AECL), and has a value of over $300 million (Cnd).
AECL's heavy water stockpile is to supply the moderator for
future sale of CANDU stations. Owing to the extreme cleanliness
requirements in SNO, the heavy water will come directly
from the Bruce plant. This is arranged by an exchange
agreement with Ontario hydro.

It is important to note that heavy water is not radioactive,
nor is it dangerous to humans or other life unless ingested in
in large amounts (it becomes toxic to humans at a level of
roughly 10% of their body weight). The deuterium
isotope occurs naturally in the ratio 1:4500; thus D2O
is found at the level of about 1 in 20 million water molecules.